1. Field of the Invention
This invention relates to a novel process for forming photographic images consisting of a silver image and a dye image, wherein the contribution of the dye image is at least 20% based on the total image, and, more particularly, to a process for obtaining suitable image density from a silver halide photographic light sensitive material which contains a relatively low amount of coated silver. The process of the present invention is particularly useful in the field of X-ray photography.
2. Discussion of the Prior Art
The increasing cost of silver halides tends to increase the production cost of photographic materials. This tendency is especially seen in the case of X-ray photographic materials since X-ray photographic materials have normally a very heavy coating weight of silver. Accordingly, various processes have been proposed directed to reducing the amount of silver while retaining the necessary image density. For example, there is a process of forming a dye image together with a metallic silver image to secure photographic images of adequate density in the sum of the both images as described in U.S. Pat. Nos. 3,615,509, 3,622,629, 2,181,944, West German Patent Nos. 1,158,836 and 537,923, British Pat. Nos. 492,518, Japanese Patent Application (OPI) Nos. 37,539/72, 4,725/77 and 57,827/77. So far these processes have not been put into practice due to the difficulty in obtaining high image density for the amount of coated silver or due to the comparatively high price of the materials for forming images.
British Pat. No. 1,122,088 has also proposed a process for the production of photographic images consisting of metallic silver together with a blue dye which comprises subjecting an imagewise exposed silver halide photographic material to development with a developer comprising 1-phenyl-3-pyrazolidone and 4-methoxy-1-naphthol. However, this process using 4-methoxy-1-naphthol is not yet satisfactory, in addition to the aforementioned disadvantages, in that color hue of the so obtained dye image is blue.
Apart from such a concept as reducing the consumption of silver, it is known to use dihydroxy naphthalene derivatives, which are structurally similar to those used in this invention, in the photographic system for quite different purposes. For example 1,3-dihydroxynaphthalenes having a carboxy or sulfo group are employed as antifoggants in U.S. Pat. No. 3,929,486 issued to Habu et al. A similar use of 1,4-dihydroxynaphthalenes as antifoggants is also described in U.S. Pat. No. 2,418,613. Further, 2,3-dihydroxynaphthalenes are described in U.S. Pat. No. 3,236,652 issued to Kennard et al and used as chelating agents. In U.S. Pat. No. 3,459,549 issued to McBride et al, bridged 1,4- and 1,2-dihydroxynaphthalenes are used as developing agents and antifoggants. While it is proposed to use such dihydroxy naphthalenes in the photographic system, these dihydroxynaphthalenes are not suitable for use in forming a dye image utilizing them, as intended in the present invention, since these naphthalenes cannot provide an appropriate electron transfer system associated with 3-pyrazolidone compounds. In more detail, where hydroxy groups are substituted at the 1- and 4-positions of the naphthalene nucleus, its developing activity is too strong and hence stability is very poor. Where hydroxy groups are substituted at the 1- and 3-positions thereof, the developing activity is rather too weak so that these dihydroxynaphthalenes cannot be oxidized sufficiently to give a dye image. In addition, the aforesaid 1,3- and 2,3-dihydroxynaphthalenes mandatorily require the presence of water-solubilizing groups such as carboxy or sulfo which would adversely affect an electron transfer system between dihydroxynaphthalenes and 3-pyrazolidones necessary for achieving the purposes of this invention. The use of the bridged dihydroxynaphthalenes is, needless to say, against the intended cost reduction due to the increasing cost reflected by complicated routes of preparing such bridged dihydroxynaphthalenes.